package aether

import (
	"errors"
	"fmt"
	"time"
)

// ErrVersionConflict is returned when attempting to save an event with a version
// that is not strictly greater than the current latest version for an actor.
// This ensures events have monotonically increasing versions per actor.
var ErrVersionConflict = errors.New("version conflict")

// VersionConflictError provides details about a version conflict.
// It is returned when SaveEvent is called with a version <= the current latest version.
type VersionConflictError struct {
	ActorID          string
	AttemptedVersion int64
	CurrentVersion   int64
}

func (e *VersionConflictError) Error() string {
	return fmt.Sprintf("%s: actor %q has version %d, cannot save version %d",
		ErrVersionConflict, e.ActorID, e.CurrentVersion, e.AttemptedVersion)
}

func (e *VersionConflictError) Unwrap() error {
	return ErrVersionConflict
}

// Event represents a domain event in the system
type Event struct {
	ID        string                 `json:"id"`
	EventType string                 `json:"eventType"`
	ActorID   string                 `json:"actorId"`
	CommandID string                 `json:"commandId,omitempty"` // Correlation ID for command that triggered this event
	Version   int64                  `json:"version"`
	Data      map[string]interface{} `json:"data"`
	Timestamp time.Time              `json:"timestamp"`
}

// ActorSnapshot represents a point-in-time state snapshot
type ActorSnapshot struct {
	ActorID   string                 `json:"actorId"`
	Version   int64                  `json:"version"`
	State     map[string]interface{} `json:"state"`
	Timestamp time.Time              `json:"timestamp"`
}

// EventStore defines the interface for event persistence.
//
// # Version Semantics
//
// Events for an actor must have monotonically increasing versions. When SaveEvent
// is called, the implementation must validate that the event's version is strictly
// greater than the current latest version for that actor. If the version is less
// than or equal to the current version, SaveEvent must return a VersionConflictError
// (which wraps ErrVersionConflict).
//
// This validation ensures event stream integrity and enables optimistic concurrency
// control. Clients should:
//  1. Call GetLatestVersion to get the current version for an actor
//  2. Set the new event's version to currentVersion + 1
//  3. Call SaveEvent - if it returns ErrVersionConflict, another writer won
//  4. On conflict, reload the latest version and retry if appropriate
//
// For new actors (no existing events), version 1 is expected for the first event.
type EventStore interface {
	// SaveEvent persists an event to the store. The event's Version must be
	// strictly greater than the current latest version for the actor.
	// Returns VersionConflictError if version <= current latest version.
	SaveEvent(event *Event) error

	// GetEvents retrieves events for an actor from a specific version (inclusive).
	// Returns an empty slice if no events exist for the actor.
	GetEvents(actorID string, fromVersion int64) ([]*Event, error)

	// GetLatestVersion returns the latest version for an actor.
	// Returns 0 if no events exist for the actor.
	GetLatestVersion(actorID string) (int64, error)
}

// SnapshotStore extends EventStore with snapshot capabilities
type SnapshotStore interface {
	EventStore
	GetLatestSnapshot(actorID string) (*ActorSnapshot, error)
	SaveSnapshot(snapshot *ActorSnapshot) error
}